A gas turbine engine powering an aircraft typically includes a fan section driven by a core engine or gas generating core. The gas generating core includes a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section through a driven shaft.
The driven shaft is required to include a structure sufficient to transmit rotation to the fan section. The power requirements of the shaft dictate the size of the shaft. The size of the shaft in turn results in a minimum diameter of core gas generating structures such as the compressor and turbine sections.
Historically, the fan section and gas generator are arranged along a common axis. The engine core must be sized such that it is built around the fan shaft. Accordingly, alternate engine configurations may be required and developed to enable improvements to thrust and engine efficiency.